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Linus Torvalds1da177e2005-04-16 15:20:36 -07001Linux Magic System Request Key Hacks
Randy Dunlapd346cce2007-01-31 23:48:17 -08002Documentation for sysrq.c
Linus Torvalds1da177e2005-04-16 15:20:36 -07003
4* What is the magic SysRq key?
5~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
6It is a 'magical' key combo you can hit which the kernel will respond to
7regardless of whatever else it is doing, unless it is completely locked up.
8
9* How do I enable the magic SysRq key?
10~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
11You need to say "yes" to 'Magic SysRq key (CONFIG_MAGIC_SYSRQ)' when
12configuring the kernel. When running a kernel with SysRq compiled in,
13/proc/sys/kernel/sysrq controls the functions allowed to be invoked via
Ben Hutchings8eaede42013-10-07 01:05:46 +010014the SysRq key. The default value in this file is set by the
15CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE config symbol, which itself defaults
16to 1. Here is the list of possible values in /proc/sys/kernel/sysrq:
Linus Torvalds1da177e2005-04-16 15:20:36 -070017 0 - disable sysrq completely
18 1 - enable all functions of sysrq
19 >1 - bitmask of allowed sysrq functions (see below for detailed function
20 description):
Ben Hutchingse8b5cbb2013-10-07 00:48:46 +010021 2 = 0x2 - enable control of console logging level
22 4 = 0x4 - enable control of keyboard (SAK, unraw)
23 8 = 0x8 - enable debugging dumps of processes etc.
24 16 = 0x10 - enable sync command
25 32 = 0x20 - enable remount read-only
26 64 = 0x40 - enable signalling of processes (term, kill, oom-kill)
27 128 = 0x80 - allow reboot/poweroff
28 256 = 0x100 - allow nicing of all RT tasks
Linus Torvalds1da177e2005-04-16 15:20:36 -070029
30You can set the value in the file by the following command:
31 echo "number" >/proc/sys/kernel/sysrq
32
Ben Hutchings8eaede42013-10-07 01:05:46 +010033The number may be written here either as decimal or as hexadecimal
34with the 0x prefix. CONFIG_MAGIC_SYSRQ_DEFAULT_ENABLE must always be
35written in hexadecimal.
Ben Hutchingse8b5cbb2013-10-07 00:48:46 +010036
Linus Torvalds1da177e2005-04-16 15:20:36 -070037Note that the value of /proc/sys/kernel/sysrq influences only the invocation
38via a keyboard. Invocation of any operation via /proc/sysrq-trigger is always
Randy Dunlapd346cce2007-01-31 23:48:17 -080039allowed (by a user with admin privileges).
Linus Torvalds1da177e2005-04-16 15:20:36 -070040
41* How do I use the magic SysRq key?
42~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
43On x86 - You press the key combo 'ALT-SysRq-<command key>'. Note - Some
44 keyboards may not have a key labeled 'SysRq'. The 'SysRq' key is
45 also known as the 'Print Screen' key. Also some keyboards cannot
46 handle so many keys being pressed at the same time, so you might
Pavel Machekdfb00422007-10-07 00:24:37 -070047 have better luck with "press Alt", "press SysRq", "release SysRq",
Linus Torvalds1da177e2005-04-16 15:20:36 -070048 "press <command key>", release everything.
49
50On SPARC - You press 'ALT-STOP-<command key>', I believe.
51
52On the serial console (PC style standard serial ports only) -
53 You send a BREAK, then within 5 seconds a command key. Sending
54 BREAK twice is interpreted as a normal BREAK.
55
56On PowerPC - Press 'ALT - Print Screen (or F13) - <command key>,
57 Print Screen (or F13) - <command key> may suffice.
58
59On other - If you know of the key combos for other architectures, please
60 let me know so I can add them to this section.
61
Randy Dunlapd346cce2007-01-31 23:48:17 -080062On all - write a character to /proc/sysrq-trigger. e.g.:
Linus Torvalds1da177e2005-04-16 15:20:36 -070063
64 echo t > /proc/sysrq-trigger
65
66* What are the 'command' keys?
67~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Linus Torvalds1da177e2005-04-16 15:20:36 -070068'b' - Will immediately reboot the system without syncing or unmounting
69 your disks.
70
Hidetoshi Setocab8bd32009-07-29 15:04:14 -070071'c' - Will perform a system crash by a NULL pointer dereference.
72 A crashdump will be taken if configured.
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -070073
Randy Dunlapd346cce2007-01-31 23:48:17 -080074'd' - Shows all locks that are held.
75
Randy Dunlap78831ba2007-02-10 01:45:55 -080076'e' - Send a SIGTERM to all processes, except for init.
Linus Torvalds1da177e2005-04-16 15:20:36 -070077
David Rientjes071a4be2015-09-08 15:00:42 -070078'f' - Will call the oom killer to kill a memory hog process, but do not
79 panic if nothing can be killed.
Linus Torvalds1da177e2005-04-16 15:20:36 -070080
Randy Dunlap003bb8a2010-10-26 14:20:40 -070081'g' - Used by kgdb (kernel debugger)
Linus Torvalds1da177e2005-04-16 15:20:36 -070082
Randy Dunlap78831ba2007-02-10 01:45:55 -080083'h' - Will display help (actually any other key than those listed
Jesper Juhl09736bd2007-08-10 13:01:04 -070084 here will display help. but 'h' is easy to remember :-)
Linus Torvalds1da177e2005-04-16 15:20:36 -070085
Randy Dunlap78831ba2007-02-10 01:45:55 -080086'i' - Send a SIGKILL to all processes, except for init.
87
Eric Sandeenc2d75432009-03-31 15:23:46 -070088'j' - Forcibly "Just thaw it" - filesystems frozen by the FIFREEZE ioctl.
89
Randy Dunlap78831ba2007-02-10 01:45:55 -080090'k' - Secure Access Key (SAK) Kills all programs on the current virtual
91 console. NOTE: See important comments below in SAK section.
Linus Torvalds1da177e2005-04-16 15:20:36 -070092
Rik van Riel5045bca2008-04-29 00:59:21 -070093'l' - Shows a stack backtrace for all active CPUs.
94
Linus Torvalds1da177e2005-04-16 15:20:36 -070095'm' - Will dump current memory info to your console.
96
Randy Dunlapd346cce2007-01-31 23:48:17 -080097'n' - Used to make RT tasks nice-able
98
Randy Dunlap78831ba2007-02-10 01:45:55 -080099'o' - Will shut your system off (if configured and supported).
100
101'p' - Will dump the current registers and flags to your console.
102
Linus Torvalds99ebcf82008-10-20 13:19:56 -0700103'q' - Will dump per CPU lists of all armed hrtimers (but NOT regular
104 timer_list timers) and detailed information about all
105 clockevent devices.
Johannes Weineracf11fa2007-03-22 00:11:18 -0800106
Randy Dunlap78831ba2007-02-10 01:45:55 -0800107'r' - Turns off keyboard raw mode and sets it to XLATE.
108
109's' - Will attempt to sync all mounted filesystems.
110
111't' - Will dump a list of current tasks and their information to your
112 console.
113
114'u' - Will attempt to remount all mounted filesystems read-only.
115
Randy Dunlap003bb8a2010-10-26 14:20:40 -0700116'v' - Forcefully restores framebuffer console
117'v' - Causes ETM buffer dump [ARM-specific]
Linus Torvalds1da177e2005-04-16 15:20:36 -0700118
Randy Dunlapd346cce2007-01-31 23:48:17 -0800119'w' - Dumps tasks that are in uninterruptable (blocked) state.
120
121'x' - Used by xmon interface on ppc/powerpc platforms.
David S. Miller916ca142012-10-16 09:34:01 -0700122 Show global PMU Registers on sparc64.
James Hogan0f6ce772015-07-15 16:17:42 +0100123 Dump all TLB entries on MIPS.
Randy Dunlapd346cce2007-01-31 23:48:17 -0800124
Randy Dunlap003bb8a2010-10-26 14:20:40 -0700125'y' - Show global CPU Registers [SPARC-64 specific]
126
Randy Dunlap3871f2f2008-12-24 16:06:57 -0800127'z' - Dump the ftrace buffer
128
Linus Torvalds1da177e2005-04-16 15:20:36 -0700129'0'-'9' - Sets the console log level, controlling which kernel messages
130 will be printed to your console. ('0', for example would make
131 it so that only emergency messages like PANICs or OOPSes would
132 make it to your console.)
133
Linus Torvalds1da177e2005-04-16 15:20:36 -0700134* Okay, so what can I use them for?
135~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
zhangwei(Jovi)e2a8b0a2013-04-30 15:28:57 -0700136Well, unraw(r) is very handy when your X server or a svgalib program crashes.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700137
zhangwei(Jovi)e2a8b0a2013-04-30 15:28:57 -0700138sak(k) (Secure Access Key) is useful when you want to be sure there is no
Randy Dunlapd346cce2007-01-31 23:48:17 -0800139trojan program running at console which could grab your password
140when you would try to login. It will kill all programs on given console,
141thus letting you make sure that the login prompt you see is actually
Linus Torvalds1da177e2005-04-16 15:20:36 -0700142the one from init, not some trojan program.
Jesper Juhl3eecd1d2006-06-25 05:48:16 -0700143IMPORTANT: In its true form it is not a true SAK like the one in a :IMPORTANT
144IMPORTANT: c2 compliant system, and it should not be mistaken as :IMPORTANT
145IMPORTANT: such. :IMPORTANT
Randy Dunlapd346cce2007-01-31 23:48:17 -0800146 It seems others find it useful as (System Attention Key) which is
Linus Torvalds1da177e2005-04-16 15:20:36 -0700147useful when you want to exit a program that will not let you switch consoles.
148(For example, X or a svgalib program.)
149
zhangwei(Jovi)e2a8b0a2013-04-30 15:28:57 -0700150reboot(b) is good when you're unable to shut down. But you should also
151sync(s) and umount(u) first.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700152
zhangwei(Jovi)e2a8b0a2013-04-30 15:28:57 -0700153crash(c) can be used to manually trigger a crashdump when the system is hung.
Hidetoshi Setocab8bd32009-07-29 15:04:14 -0700154Note that this just triggers a crash if there is no dump mechanism available.
Hariprasad Nellitheertha86b1ae32005-06-25 14:58:25 -0700155
zhangwei(Jovi)e2a8b0a2013-04-30 15:28:57 -0700156sync(s) is great when your system is locked up, it allows you to sync your
Linus Torvalds1da177e2005-04-16 15:20:36 -0700157disks and will certainly lessen the chance of data loss and fscking. Note
158that the sync hasn't taken place until you see the "OK" and "Done" appear
159on the screen. (If the kernel is really in strife, you may not ever get the
160OK or Done message...)
161
zhangwei(Jovi)e2a8b0a2013-04-30 15:28:57 -0700162umount(u) is basically useful in the same ways as sync(s). I generally sync(s),
163umount(u), then reboot(b) when my system locks. It's saved me many a fsck.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700164Again, the unmount (remount read-only) hasn't taken place until you see the
165"OK" and "Done" message appear on the screen.
166
Randy Dunlapd346cce2007-01-31 23:48:17 -0800167The loglevels '0'-'9' are useful when your console is being flooded with
168kernel messages you do not want to see. Selecting '0' will prevent all but
Linus Torvalds1da177e2005-04-16 15:20:36 -0700169the most urgent kernel messages from reaching your console. (They will
170still be logged if syslogd/klogd are alive, though.)
171
zhangwei(Jovi)e2a8b0a2013-04-30 15:28:57 -0700172term(e) and kill(i) are useful if you have some sort of runaway process you
Linus Torvalds1da177e2005-04-16 15:20:36 -0700173are unable to kill any other way, especially if it's spawning other
174processes.
175
zhangwei(Jovi)e2a8b0a2013-04-30 15:28:57 -0700176"just thaw it(j)" is useful if your system becomes unresponsive due to a frozen
Eric Sandeenc2d75432009-03-31 15:23:46 -0700177(probably root) filesystem via the FIFREEZE ioctl.
178
Linus Torvalds1da177e2005-04-16 15:20:36 -0700179* Sometimes SysRq seems to get 'stuck' after using it, what can I do?
180~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
181That happens to me, also. I've found that tapping shift, alt, and control
182on both sides of the keyboard, and hitting an invalid sysrq sequence again
Randy Dunlapd346cce2007-01-31 23:48:17 -0800183will fix the problem. (i.e., something like alt-sysrq-z). Switching to another
Linus Torvalds1da177e2005-04-16 15:20:36 -0700184virtual console (ALT+Fn) and then back again should also help.
185
186* I hit SysRq, but nothing seems to happen, what's wrong?
187~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Paul Foxa2056ff2010-05-19 10:11:13 -0700188There are some keyboards that produce a different keycode for SysRq than the
189pre-defined value of 99 (see KEY_SYSRQ in include/linux/input.h), or which
190don't have a SysRq key at all. In these cases, run 'showkey -s' to find an
191appropriate scancode sequence, and use 'setkeycodes <sequence> 99' to map
192this sequence to the usual SysRq code (e.g., 'setkeycodes e05b 99'). It's
193probably best to put this command in a boot script. Oh, and by the way, you
194exit 'showkey' by not typing anything for ten seconds.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700195
196* I want to add SysRQ key events to a module, how does it work?
197~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
198In order to register a basic function with the table, you must first include
199the header 'include/linux/sysrq.h', this will define everything else you need.
200Next, you must create a sysrq_key_op struct, and populate it with A) the key
201handler function you will use, B) a help_msg string, that will print when SysRQ
202prints help, and C) an action_msg string, that will print right before your
Adrian Bunk338cec32005-09-10 00:26:54 -0700203handler is called. Your handler must conform to the prototype in 'sysrq.h'.
Linus Torvalds1da177e2005-04-16 15:20:36 -0700204
Randy Dunlapd346cce2007-01-31 23:48:17 -0800205After the sysrq_key_op is created, you can call the kernel function
206register_sysrq_key(int key, struct sysrq_key_op *op_p); this will
207register the operation pointed to by 'op_p' at table key 'key',
208if that slot in the table is blank. At module unload time, you must call
209the function unregister_sysrq_key(int key, struct sysrq_key_op *op_p), which
Linus Torvalds1da177e2005-04-16 15:20:36 -0700210will remove the key op pointed to by 'op_p' from the key 'key', if and only if
211it is currently registered in that slot. This is in case the slot has been
212overwritten since you registered it.
213
214The Magic SysRQ system works by registering key operations against a key op
215lookup table, which is defined in 'drivers/char/sysrq.c'. This key table has
216a number of operations registered into it at compile time, but is mutable,
Randy Dunlapd346cce2007-01-31 23:48:17 -0800217and 2 functions are exported for interface to it:
218 register_sysrq_key and unregister_sysrq_key.
219Of course, never ever leave an invalid pointer in the table. I.e., when
220your module that called register_sysrq_key() exits, it must call
221unregister_sysrq_key() to clean up the sysrq key table entry that it used.
222Null pointers in the table are always safe. :)
Linus Torvalds1da177e2005-04-16 15:20:36 -0700223
224If for some reason you feel the need to call the handle_sysrq function from
225within a function called by handle_sysrq, you must be aware that you are in
226a lock (you are also in an interrupt handler, which means don't sleep!), so
227you must call __handle_sysrq_nolock instead.
228
Andy Whitcroft47c33d92009-01-15 13:50:51 -0800229* When I hit a SysRq key combination only the header appears on the console?
230~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
231Sysrq output is subject to the same console loglevel control as all
232other console output. This means that if the kernel was booted 'quiet'
233as is common on distro kernels the output may not appear on the actual
234console, even though it will appear in the dmesg buffer, and be accessible
235via the dmesg command and to the consumers of /proc/kmsg. As a specific
236exception the header line from the sysrq command is passed to all console
237consumers as if the current loglevel was maximum. If only the header
238is emitted it is almost certain that the kernel loglevel is too low.
239Should you require the output on the console channel then you will need
240to temporarily up the console loglevel using alt-sysrq-8 or:
241
242 echo 8 > /proc/sysrq-trigger
243
244Remember to return the loglevel to normal after triggering the sysrq
245command you are interested in.
246
Linus Torvalds1da177e2005-04-16 15:20:36 -0700247* I have more questions, who can I ask?
248~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Randy Dunlap21106b02012-03-30 13:37:06 -0700249Just ask them on the linux-kernel mailing list:
250 linux-kernel@vger.kernel.org
Linus Torvalds1da177e2005-04-16 15:20:36 -0700251
252* Credits
253~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Adam D. Moss5e03e2c2006-01-03 13:31:01 +0100254Written by Mydraal <vulpyne@vulpyne.net>
Linus Torvalds1da177e2005-04-16 15:20:36 -0700255Updated by Adam Sulmicki <adam@cfar.umd.edu>
256Updated by Jeremy M. Dolan <jmd@turbogeek.org> 2001/01/28 10:15:59
257Added to by Crutcher Dunnavant <crutcher+kernel@datastacks.com>